Systemic lupus erythematosus (SLE) is characterized by abnormal T-cell activation and death, processes which are crucially dependent on the controlled production of reactive oxygen intermediates (ROI) and of ATP in mitochondria. The mitochondrial transmembrane potential has conclusively emerged as a critical checkpoint of ATP synthesis and cell death. In normal T cells, we firstly identified the elevation of the mitochondrial transmembrane potential, i.e., mitochondrial hyperpolarization (MHP), and, secondly, also ATP depletion, which are early and reversible steps of T-cell activation and apoptosis. Conversely, in SLE patients, we found that T cells exhibit persistent MHP as well as ATP and glutathione depletion which decrease activation-induced apoptosis and instead predispose T cells for necrosis, thus stimulating inflammation in SLE. Therefore, determining the molecular basis and consequences of persistent MHP is essential for understanding the mechanism of altered activation and death signaling in lupus T cells. We found persistent MHP to be associated with increased mitochondrial mass and increased mitochondrial and cytoplasmic Ca2+ content in T lymphocytes and also with enhanced nitric oxide (NO) production in monocytes. NO-induced mitochondrial biogenesis in normal T cells accelerates the rapid phase and reduces the plateau of Ca2+ influx upon CD3/CD28 costimulation, thus mimicking the Ca2+ signaling profile of lupus T cells. Since mitochondria are major Ca2+ stores, NO-dependent mitochondrial biogenesis may account for altered Ca2+ handling. In lupus T cells, we identified changes in expression of genes that control key metabolic pathways: over-expression of transaldolase (TAL) which induces glutathione depletion and MHP, low expression of eNOS-interacting protein (NOSIP) that regulates compartmentalized production of NO, and over-expression of the rapamycin receptor FKBP12. We observed improvement of disease activity, normalization of CD3/CD28-induced Ca2+ fluxing, and persistence of MHP in rapamycin-treated patients, suggesting that altered Ca2+ fluxing is downstream or independent of mitochondrial dysfunction. The proposed studies will test the hypothesis that inhibition of the electron transport chain via S-nitrosylation stemming from glutathione depletion in the presence of NO causes persistent MHP which, in turn, activates the mammalian target of rapamcyin (mTOR) pathway. First, we will measure functional capacity of the electron transport chain in isolated mitochondria and determine the role of GSH depletion and TAL activation in MHP and ATP depletion of lupus T cells. Second, we will determine the role of intrinsic and extrinsic NO production, compartmentalized expression of eNOS, and responsiveness to NO. Third, we will examine the role of mTOR as a sensor and down-stream effector of MHP and controller of increased Ca2+ fluxing. Fourth, we will systematically map metabolic checkpoints upstream and downstream of MHP and validate the involvement of candidate genes that can be targeted to normalize T-cell activation in SLE.